Thermophotovoltaic (TPV) energy systems convert thermal energy to electric power using the same principle of operation as solar cells. In particular, a heat source radiatively emits photons that are incident on a semiconductor TPV device with an energy (E) spectrum that can be characterized by Planck's blackbody distribution modified to account for the radiator surface spectral emissivity. The wavelength (λ) of a photon is inversely proportional to its photon energy and can be calculated from λ=hc/E where h is Planck's constant and c is the speed of light. Photons with energy greater than the semiconductor bandgap (Eg) (typically ranging from 0.50 to 0.74 eV for TPV devices) excite electrons from the valence band to the conduction band of the semiconductor material (interband transition). The resultant electron-hole pairs are then collected and can be used to power electrical loads. Photons with energy less than the semiconductor bandgap cannot be converted to electrical energy and, therefore, are parasitically absorbed as heat. In order to increase the conversion efficiency of a TPV system, some form of spectral control is employed to reduce the amount of below bandgap energy that is parasitically absorbed.
Various spectral control methods have been proposed to improve TPV conversion efficiency. In one method, the emission spectrum of the photon radiator is changed to suppress emission of below bandgap energy. Several techniques have been used including surface texturing and rare earth oxide coatings. U.S. Pat. No. 4,764,104 to Nelson reports a narrow band thermally energized radiation source composed of a rare earth oxide radiator member. When heated to about 1700 degrees C., the rare earth oxide radiator member has a concentrated radiated flux over the 0.4 to 2.5 micrometer wavelength range such that at least 50% of the radiated flux is within a spectral band that is less than 0.4 micrometers wide. The significant issue with this type of selective radiator for some applications is the reduction of above bandgap energy incident on the TPV device compared with TPV systems that employ high emissivity blackbody radiators. This reduction in available above bandgap energy causes a reduced electrical output surface power density.
In another method, a highly reflective coating is applied to the back surface of the TPV device. The TPV device is designed such that the above bandgap (useful) energy is absorbed in the active layers of the TPV device. The TPV device is further designed such that most of the below bandgap (useless) energy passes through the TPV device and is reflected back to the radiator after passing through the TPV device a second time. This method was reported by An-Ti Chai, Back Surface Reflection for Solar Cells, the Fourteenth IEEE Photovoltaics Specialists Conference, 1980. A critical issue associated with this approach is the amount of below bandgap energy parasitically absorbed during transit through the TPV device.
In yet another method, a selective filter is placed in front of the TPV device. The filter is designed to transmit most of the above bandgap energy to the TPV device and reflect most of the below bandgap energy back to the radiator. The effectiveness of a TPV device is a strong function of the filter's ability to minimize parasitic absorption by achieving a very high reflection of below bandgap energy as well as minimizing absorption of above bandgap energy in the filter. This spectral control method is described in An Experimental Assessment of Low Temperature Voltaic Energy Conversion, PF Baldasaro et al., First NREL Conference on Thermophotovoltaic Generation of Electricity, Jul. 24–27, 1994.
In a TPV energy conversion system, spectral control via selective filters requires a filter (or filter system) with certain characteristics. First, there should be a high transmission of energy above the TPV device semiconductor bandgap (energy with wavelength less than the wavelength corresponding to the TPV device semiconductor bandgap (λg) which is equal to 1.24/Eg where the unit for Eg is electron-volts and the unit for wavelength is micrometers). Second, there should be a high reflection of energy below the TPV device semiconductor bandgap (energy with wavelength greater than λg). Third, there should be a sharp transition from high transmission (low reflection) to high reflection at the wavelength corresponding to the TPV cell semiconductor bandgap. Fourth, there should be minimal absorption of above bandgap energy in the filter itself.
In general, no single filter has been identified that meets all of these characteristics. Interference filters are multi-layer stacks of non-absorbing dielectric or semiconductor materials with alternating high and low indices of refraction. The measured performance of a typical interference filter used in a thermophotovoltaic system is illustrated in FIG. 1 wherein the typical interference filter's reflectivity is shown as a function of incident wavelength. As shown in FIG. 1, the typical interference filter demonstrates an abrupt transition between the low reflection and high reflection regions. As further shown in FIG. 1, the reflection zone of the typical interference filter extends to approximately 6 micrometers. In most TPV systems, 25% of the blackbody radiation is emitted with wavelengths greater than the reflective range of a practical interference filter, therefore, an interference filter alone does not meet all of the desired filter characteristics described above.
A plasma filter is a single layer of material (typically a semiconductor material) with a high concentration of free charge carriers (typically greater than 5×1019 carriers per cm3). The concentration of free charge carriers (N) present in a plasma filter determines the plasma wavelength, λp. For N˜5×1019 cm−3, λp is approximately 5 micrometers for typical semiconductor plasma filters. In general, plasma filters provide high transmission of energy with wavelength less than λp, and high reflection for energy with wavelength greater than λp However, plasma filters exhibit an absorption peak that occurs at the plasma wavelength and, therefore, do not exhibit a sharp transition from high transmission to high reflection. Therefore, a plasma filter with plasma wavelength approximately equal to the semiconductor device bandgap would provide high reflection for below bandgap energy, but would also exhibit a large amount of parasitic absorption of above bandgap energy. Shifting the plasma wavelength of the plasma filter to longer wavelengths reduces the parasitic absorption of above bandgap energy, but also reduces the reflectivity of shorter wavelength below bandgap energy.
The measured performance of a typical plasma filter is illustrated in FIG. 2 wherein the typical plasma filter's reflectivity is shown as a function of incident wavelength. As shown in FIG. 2, the typical plasma filter shows a gradual transition between the low reflection and high reflection regions. Further, the high reflection zone of the typical plasma filter extends into the long wavelength regions. As shown in FIG. 2, the typical plasma filter alone does not meet all of the desired filter characteristics described above.
Previous researchers have proposed the combination of an interference filter with a plasma filter in various tandem arrangements to overcome the limitations of each component. U.S. Pat. No. 4,017,758 to Almer, suggested this solution for application in incandescent light bulbs in 1977. U.S. Pat. No. 5,403,405 to Fraas, et al., proposed this concept for application in a TPV energy conversion system in 1993. This combination of filters was improved upon in U.S. Pat. No. 5,700,332 to Brown, et al., by providing a physical separation between the interference filter and the plasma filter to provide improved performance of the filter system. U.S. Pat. No. 6,043,426 to DePoy, et al., further improved upon this concept by incorporating the plasma filter as the top layer in the TPV device.
FIG. 3 illustrates the measured reflection versus wavelength for a typical high performance tandem filter that combines an interference filter and a plasma filter. For this tandem filter concept, the interference filter is located on top of the plasma filter (between the plasma filter and the radiator). The plasma filter has a plasma wavelength of approximately 5 micrometers and provides high reflection of long wavelength below bandgap energy. The plasma filter also exhibits very low parasitic absorption of above bandgap energy. The interference filter provides high transmission of above bandgap energy, high reflection of short wavelength below bandgap energy and a sharp transition from high transmission to high reflection. Each filter compensates for the weaknesses of the other, and, taken together, the filter system satisfies each of the requirements listed above.
A third type of filter is called a frequency selective surface (FSS). A frequency selective surface is a two-dimensional periodic array of electromagnetic scattering elements. These elements consist of either patches (isolated metal elements) or apertures (holes in a metal layer). The spectral performance (reflection and transmission versus wavelength) of an FSS is governed by the size, shape, and spacing of the electromagnetic scattering elements. U.S. Pat. No. 5,611,870 to Horne and Morgan, proposed the use of a frequency selective surface as a filter array for modifying radiant thermal energy in photovoltaic and thermophotovoltaic systems. U.S. Pat. No. 5,861,226 to Horne and Morgan, also proposed a method of fabricating the filter array (FSS) using a masked ion beam lithography process.
FSS filters typically provide excellent reflection of long wavelength below bandgap energy. However, a performance tradeoff exists between the transmission of above bandgap energy and the reflection of short wavelength below bandgap energy. FSS filters that have high transmission of above bandgap energy typically do not provide the desired sharp transition to high reflection and, therefore, do not have high reflection of short wavelength below bandgap energy.
FIG. 4 illustrates the reflection versus wavelength of a typical frequency selective surface designed to provide high reflection of all below bandgap energy. Although this filter provides high reflection of all below bandgap energy, it suffers from very low integrated transmission of above bandgap energy caused by high above bandgap reflection. Therefore, frequency selective surfaces designed to maximize reflection of below bandgap energy do not meet all of the desired filter characteristics described above.
FIG. 5 illustrates the predicted performance of a typical frequency selective surface that has been designed to provide low reflection (high transmission) of all above bandgap energy. Although this filter provides high transmission of above bandgap energy, it does not exhibit an abrupt transition from low reflection (high transmission) to high reflection and, therefore, does not provide high reflection for short wavelength below bandgap energy. Therefore, frequency selective surfaces designed to maximize transmission of above bandgap energy do not meet all of the desired filter characteristics described above.